Phosphating ferrous substrates to produce an iron phosphate coating is an old and well-known art. Such coatings are typically employed to pretreat the surface of ferrous substrates prior to applying other protective coatings and aid in resisting corrosion.
In general, there are two types of phosphating processes well-known to those skilled in the art. One such process utilizes a bath solution which contains heavy metal ions such as iron, zinc and the like. This process is generally understood in the art to form an essentially crystalline coating wherein heavy metal ions initially present in the bath form a part of the coating. Examples of such processes are disclosed in prior U.S. Pat. Nos. 4,670,066; 4,474,626 and 4,728,373.
The other type of well-known phosphating process, sometimes referred to as the alkali metal phosphate process, utilizes a bath solution which contains no heavy metal ions and an essentially amorphous iron phosphate coating is formed which is generally satisfactory in applications wherein a relatively light or thin coating is acceptable. While such a coating includes a variable percentage of iron oxide, it is generally referred to as an iron phosphate coat by those skilled in the art.
It is also recognized by those skilled in the art that the mechanisms of coating formation is obviously different between the alkali metal phosphate system and the heavy metal phosphate system. The crystalline phosphate coating formed in the heavy metal type process is generally adherent and the coating thickness can be relatively heavy or thick without significant loss of its adherent nature. However, the heavy metal ions present in the bath composition represent a very difficult environmental waste disposal problem compared to the alkali metal phosphating systems.
With respect to the alkali metal systems, presently the two most often used methods of applying an essentially amorphous iron phosphate coating to substrates are a spraying process and an immersion process. One drawback to the spraying process is that it is typically limited to a relatively light or intermediate iron phosphate coating. A light coating is generally considered by those skilled in the art as about 35 mg per square foot or less. Intermediate coatings range from 35 mg to about 80 mg per square foot. The immersion process also provides a reasonably satisfactory iron phosphate coating for application of light and intermediate coatings. However, as one approaches the upper portion of this intermediate range, about 60 to 70 mg per square foot, the iron phosphate coatings tend to become less tightly held or bound to the substrate and "dusting" occurs. "Dusting" as referred to herein means an iron phosphate coat which is powdery and so poorly adherent or loosely bound to the substrate that the coating tends to become easily lost from the surface of the substrate. For those coatings referred to as heavy coatings by those in this field, that is above about 80 mg per square foot, the formation of a powdery coating becomes sufficiently severe as to extremely limit use of such coatings for many applications. Further, in some instances such loosely held, powdery coatings represent a serious health hazard in the workplace as iron phosphate dust can become airborne and may effect personnel working in the exposed area.
Prior to the present invention, it was well known to those skilled in the art that chlorate and organic accelerated alkali metal phosphating immersion baths produced heavy coatings, however, such coatings were loosely adhered particularly near the surface of the coatings and considered unsatisfactory. Inorganic accelerated baths tend to produce only relatively lighter coatings on a practical or cost efficient basis.
It has long been recognized that a thicker or heavier iron phosphate coating produced using a bath which contains no heavy metal ions is highly desirable to improve corrosion resistance of the substrate with or without the application of an additional protective coating, such as a paint or oil coat. However, poorly adherent, powdery coatings are generally unacceptable as a paint pre-treatment step due to the poor adhesion of the paint to the coating. Further, loosely held particles of the phosphate coating which are easily dislodged from the surface during handling and the like, represent a significant waste of the deposited coating and reduce the effectiveness of the coating process. The deposition of a satisfactory, heavy, amorphous iron phosphate coating useful for a wide variety of applications has been a significant and long standing problem to those skilled in the art.
The immersion process is recognized as the preferred method to apply a heavier phosphate coat because of the better control of time and consistency of the contact between the substrate and the phosphating bath, as well as more cost effective control of the other operative bath parameters. However, prior to the present invention, a satisfactory immersion bath composition, of the alkali metal type which provides a heavy, strongly adherent, non-powdery, amorphous iron phosphate coat has eluded those skilled in the art.